First and foremost, farming a Stone of Jordan isn't the most reliable, as it can take you some time.
Your character level does not affect what loot is dropped, only the monster's level. However, your Magic Find can greatly affect what loot is dropped.
If you are looking for this specific drop, Nightmare Andariel (final boss of act 1, in Nightmare difficulty) has the highest drop rate. She has a better chance of dropping rings than some other bosses, and she is the appropriate level to drop a Stone of Jordan instead of a different unique ring (there are fewer variations of rings that she can drop due to her level).
This said, you won't get nearly as much other loot just farming Nightmare Andariel. If you are looking to have a more beneficial farming experience whilst looking for a Stone of Jordan, I suggest farming a different boss, like Hell Andariel, Nightmare Mephisto, or Hell Mephisto. Though you will see a lower drop chance for this ring, you will see plenty of other useful loot while you are looking for it.
Be aware that looking for this specific drop, even off of Nightmare Andariel, you may be killing her over a thousand times before you see this ring drop, if you see this ring drop.
- Lots of personal Diablo II playtime.
- Here is a Stone of Jordan farming guide.
- And another.